This invention relates generally to the field of continuous ink jet print head design. More specifically, it relates to improving print resolution by redesigning the ink flow patterns emanating from printhead nozzles.
Traditionally, digitally controlled ink jet printing capability is accomplished by one of two technologies. Typically, ink is fed through channels formed in a printhead. Each channel includes a nozzle from which ink drops are selectively ejected and deposited upon a medium.
The first technology, commonly referred to as xe2x80x9cdrop on demandxe2x80x9d ink jet printing, provides ink drops for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a drop that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle.
The second technology, commonly referred to as xe2x80x9ccontinuous streamxe2x80x9d or xe2x80x9ccontinuousxe2x80x9d ink jet printing, uses a pressurized ink source which produces a continuous stream of ink drops. Conventional continuous inkjet printers utilize electrostatic charging devices that are placed close to the point where a filament of working fluid breaks into individual ink drops. The ink drops are electrically charged and then directed to an appropriate location by deflection electrodes having a large potential difference. When no print is desired, the ink drops are deflected into an ink capturing mechanism (catcher, interceptor, gutter, etc.) and either recycled or disposed of. When a print is desired, the ink drops are not deflected and are thereby allowed to strike a print media. Alternatively, deflected ink drops may be allowed to strike the print media, while non-deflected ink drops are collected in the ink capturing mechanism.
U.S. Pat. No. 6,079,821, issued to Chwalek et al., on Jun. 27, 2000, discloses a continuous ink jet printer that uses actuation of asymmetric heaters to create individual ink drops from a filament of working fluid and deflect those ink drops. A printhead includes a pressurized ink source and an asymmetric heater operable to form printed ink drops and non-printed ink drops. Printed ink drops flow along a printed ink drop path ultimately striking a print media, while non-printed ink drops flow along a non-printed ink drop path ultimately striking a catcher surface. Non-printed ink drops are recycled or disposed of through an ink removal channel formed in the catcher.
Traditionally, ink jet nozzles for both xe2x80x9cdrop on demandxe2x80x9d and xe2x80x9ccontinuousxe2x80x9d ink jet printheads are formed in an array or row, often a linear array or row, and fixed in a single plane, the nozzles in a row being equally spaced. A row of nozzles is comprised of xe2x80x9cend nozzlesxe2x80x9d (commonly referred to as end jets, etc.) which are nozzles at each end of the row, and xe2x80x9cinner nozzlesxe2x80x9d positioned inside the end nozzles within the row. The ink streams and ink drops ejected from end nozzles and inner nozzles, respectively, are referred to as end streams and end drops and as inner streams and inner drops, respectively. As such, one would expect the pattern of printed ink drops 20, printed on a recording medium 22, to mirror the pattern of the nozzles of the linear array, as shown in FIG. 1a. However, it has been observed that ink stream flow patterns of end nozzles are out-of-line or incongruent when compared to ink stream flow patterns of inner nozzles, resulting in a failure of the pattern of printed ink drops 20, printed on a recording medium 22, to mirror the pattern of the nozzles of the linear array. Referring to FIG. 1b printed ink drops 21, ejected from end nozzles, are printed on the recording medium at a location displaced perpendicularly relative to other printed ink drops 20, ejected from inner nozzles. This perpendicular direction is commonly referred as a xe2x80x9cfast scanxe2x80x9d direction, since many commercial printers scan the printhead rapidly over a recording medium in this direction to print a pattern of drops known as an image swath. The reduction in ink drop placement accuracy degrades the printing performance of the end nozzles and of the printhead. Additionally, ink drop misplacement in the fast scan direction causes a reduction in overall image print quality.
It was theorized in the late 1970""s and early 80""s that this problem in print resolution stemmed from the fact that ink drops or ink streams ejected from end nozzles, positioned at an end of the nozzle array, were exposed to the ambient air, more so than ink drops or ink streams ejected from inner nozzles, positioned within the nozzle array. Ink ejected form end nozzles was thought to be subjected to aerodynamic drag, a force directed in a line along the trajectory of the ink drops but opposing their motion. This force reduced the velocity of streams of ink or ink drops ejected from end nozzles relative to the velocity of ink streams or ink drops ejected from inner nozzles. Thus, ink drops 21 ejected from end nozzles were caused to strike the print medium 22 at a later time than ink drops 20 ejected from inner nozzles. The resultant printed image of printed ink drops ejected from a linear array of nozzles was curved rather than in a straight line (see FIG. 1b), as desired, thus creating image artifacts and reducing image resolution. Such aerodynamic drag could reduce resolution in all inkjet printers including drop on demand and continuous ink jet printers.
In order to improve print resolution, various efforts were directed toward compensating for the velocity reduction due to aerodynamic drag. A substantially uniform line of ink drops from all of the in-line nozzles of the multi-nozzle array, was desired, and it was reasoned the if end drops could be made to strike the recording medium at the desired location by compensation for drag, higher print resolution would result.
Methods for correcting the printed location of end drops have been disclosed in xe2x80x9cReducing Drop Misregistration from Differential Aerodynamic Retardation in a Linear Ink Jet Array,xe2x80x9d IBM Technical Disclosure Bulletin, Volume 17, No. 10 by D. E. Fisher and D. L. Sipple as early as March of 1975. One correction method used control algorithms to vary the time of flight of drops from the nozzle to the recording medium and thus to cause an ink stream curvature opposite to that caused by the aerodynamic drag. A method set forth for correcting the effects of aerodynamic drag was to use a compensating velocity across the array. Alternatively, a decreased path length was found to similarly compensate.
U.S. Pat. No. 3,562,757, issued to Bischoff, corrected for drag on a drop-to-drop basis. Every other drop was guttered thereby increasing the distance between drops used for printing so that the all drops experienced some drag.
U.S. Pat. No. 3,596,275, issued to Sweet, disclosed use of an extraneous collinear stream of air with the stream of ink drops to reduce the effects aerodynamic drag. A fan, or the like, was necessary to generate the airflow.
U.S. Pat. No. 4,077,040, issued to Hendriks, reduced the effect of aerodynamic retardation or drag between streams by utilizing drop streams on the perimeter of the array which were never printed but instead continually guttered to produce a counter airflow tending to reduce retardation of drop streams emitted from the other nozzles.
U.S. Pat. No. 4,185,290, issued to Hoffman, caused each of the streams of drops ejected from end nozzles at each end of the array to have an initial velocity higher than the initial velocity of the streams of drops ejected form inner nozzles inside the end nozzles of the array, thereby compensating for the aerodynamic drag on ink streams at the end of the array. The higher initial velocity of drops ejected from the end nozzles was made possible by changing the length of the longitudinal passages in those nozzles.
Recently, continuous ink jet print heads have been made with increased nozzle densities, for example nozzle densities of 1200 nozzles per inch and higher. As nozzle densities and printing speeds have increased, the ability to reduce image artifacts and to achieve finer resolution, by merely compensating for the aerodynamic drag on ink streams at the end of the array, has proven insufficient. The difficulties have arisen, in part because, higher density printing gives rise not only to a need for correcting displacement of ink drops in the fast scan direction, shown in FIG. 1b, but also to a need for correcting displacement of ink drops perpendicular to the fast scan direction, that is, in a slow scan direction, as shown in FIG. 1c. The term slow scan direction is known and used in the art of commercial desktop printer design. In most desktop printers, the printhead is first scanned rapidly in the fast scan direction to print an image swath, then stepped or moved a small amount in a direction perpendicular to the fast scan direction (the slow scan direction) before another fast scan is repeated to print a subsequent image swath.
Referring to FIG. 1d, an example of misalignment of printed ink drops in the slow scan direction, often encountered when printing with a high-density line of ink jet nozzles, is shown. An ink jet print head 24 includes a nozzle plate 26 having an array of inner nozzles 38 and end nozzles 36 each spaced apart equally one from another by a predetermined spacing D. Typically, spacing D is small in a high density nozzle row, for example 30 microns or less. Printhead 24 ejects ink 30 from an ink delivery channel 33 through nozzles 36 and 38 onto a recording medium 22. Initially, the ink 30 is ejected in the form of an ink streams 32a, 32b which subsequently breaks into or forms a stream of individual ink drops 34. Ideally, ink drops 34 travel to recording medium 22 and form printed drops 20 by impinging on recording medium 22 in a substantially equally spaced straight line (shown in FIG. 1a).
However, as shown in FIG. 1d and FIG. 1c, printed ink drops 23 printed from end nozzles 36 suffer displacement 40 (commonly referred to as misalignment, misdirection, etc.) in the slow scan direction, particularly in high density inkjet printers. In other words, ink 30 ejected from an end nozzle 36 is deflected toward an adjacent inner nozzle 38. Ink drops 34 from end nozzle 36 and adjacent inner nozzle 38 impinge on a recording medium 22 in close proximity, in particular they are spaced closer than D, by an amount E, whereas ink drops 34 from any two adjacent inner nozzles 38 impinge on recording medium 22 and are spaced a distance D apart. Thus the spacing E represents the amount of misalignment of the printed drop from end nozzle 36 and is typically a fraction of D. In some cases, misalignment in the slow scan direction can even cause ink streams 32a, 32b or ink drops 34 ejected from end nozzles to collide with drops ejected from adjacent nozzles prior to impinging on recording medium 22, causing additional image artifacts.
The initial stream trajectory 50 of all ink steams 32 in FIG. 1d is shown pointing vertically, including end nozzle 36. The initial stream trajectory 50 is defined as the average stream velocity at the base of the stream as the stream exits the nozzle. Initial stream trajectory 50 depends only on the geometry of the nozzles 36 or 38 and on the geometry of the printhead 24 at or below nozzle plate 36. If no other forces acted on ink streams 32a, 32b and ink drops 34; then, for an initial stream trajectory 50 which is vertical, the ink drops 34 would travel vertically in FIG. 1d. 
Misalignment of ink drops in the slow scan direction can be explained by examining the forces acting on each ink stream 32a, 32b and associated ink drops 34 as they travel to recording medium 22. In particular, misalignment in the slow scan direction can be explained as an imbalance between interactive forces F1 and F2, shown in FIG. 1d, acting upon an end nozzle 36, in comparison with a balance between interactive forces F1 and F2, acting upon an inner nozzle 38. Forces F1 and F2 are caused by the pressure of air surrounding each ink stream 32a, 32b and associated ink drops 34. Force F1 acts on a given ink stream 32a, 32b and ink drops 34 in a direction left, as viewed in FIG. 1d, and is caused, as will be explained, by air currents to the right of that ink stream. Force F2 acts on a given ink stream 32a, 32b and ink drops 34 in a direction right, as viewed in FIG. 1d, and is caused by air currents to the left of that ink stream. The air currents cause a deviation of the air pressure from its atmospheric pressure value according to principles to be described. For inner nozzles 38, the air currents producing forces F1 and F2 on any given ink stream 32a, derive from the motion of the right and left neighboring ink streams 32a, 32b, respectively. For inner nozzles 38, F1 and F2 are essentially identical and hence produce no net force F1-F2. For end nozzle 36 shown in FIG. 1d, the air currents producing force F2 derive from the motion of the left neighboring ink stream 32a and the value of F2 for end nozzle 36 is not too different from the value of F2 associated with an inner nozzle 38. However, the air currents producing force F1 for end nozzle 36 are different from those associated with an inner nozzle 38, since there is no stream to the right of end nozzle 36. For end nozzle 36, F1 and F2 are not identical and hence there is a net force F1-F2. As will be explained quantitatively, F1 for the end nozzle 36 exceeds F1 for the inner nozzles 38. The force F1 associated with the right most ink stream in FIG. 1d is therefore represented as a longer arrow and the net force F1-F2 on end nozzle 36 is directed left.
When interactive forces F1 and F2 are balanced, for example in the case of an inner nozzle 38, such that there is no net force on the ink stream 32a or ink drops 34, the ink stream 32a and ink drops 34 remain undeflected in the slow scan direction (left-right in FIG. 1d) and a desired printed ink drop 20 spacing is maintained. When interactive forces F1 and F2 are unbalanced, for example in the case of end nozzle 36, such that there is a net force directed left on the ink stream 32b and on the ink drops 34 ejected from end nozzle 36, the ink stream 326 and ink drops 34 are deflected left in the slow scan direction (left in FIG. 1d) and the desired printed ink drop 23 spacing is not maintained. Thus, because there is no nozzle on the other side of end nozzle 36, ink drops 34 ejected by end nozzle 36 are misdirected and land on printed locations displaced from a desired location shown at 40. The trajectory followed by ink stream 32b and ink drops 34 ejected by end nozzle 36 curves continuously from the end nozzle 36 to recording medium 22 because the forces F1 and F2 are unbalanced all along the trajectory, as will be discussed. It is important to note that misalignment of printed drops due to this curved trajectory is distinct from the hypothetical case which would occur if the interactive forces were balanced but the ejected stream was initially misdirected by a mechanism inherent in the printhead, for example by virtue of a physical manufacturing defect, in a direction left of vertical in FIG. 1d. In such a case, the drops so ejected would also fail to land at the desired location, but the trajectory would be straight.
Interactive forces F1 and F2 act on each member of a given pair of ink streams 32a, 32b to determine their trajectories and in so doing also determine the air volume between them. For example, for the second and third streams from the right in FIG. 1d, both ejected from inner nozzles (inner streams 32a), the balanced forces F1 and F2 influence the trajectories of each stream to be straight lines and thus create a balanced air volume 42 between the second and third streams. This balanced air volume is the same for all pairs of adjacent inner streams 32a, and in these cases, the printed ink drops 20 are not misaligned. For the case of two adjacent ink streams 32a, 32b one of which is ejected from end nozzle 32b (end stream 32b) and the other of which is ejected from inner nozzle 38 (inner stream 32a), such as the first and second streams from the right in FIG. 1d, the forces F1 and F2 are unbalanced. Unbalanced forces F1 and F2 alter the trajectory on the end ink stream 32b ejected from end nozzle 36 and thus create an unbalanced air volume 44, causing the printed ink drops 23 to be misaligned (location 40) in the slow scan direction by an amount E. Because of the shape of the unbalanced air volume 44 to the left side of end nozzle 36, the force F2 on the end stream 32b (first stream on the right in FIG. 1d) is slightly larger than the force F2 on inner nozzles 38 having balance air volumes to their left sides. The force F2 acting on end stream 32b is slightly larger than the force F2 on inner streams 32a ejected from inner nozzles 38 because the unbalanced air volume 44 provides a greater separation between the end stream 32b and the neighboring inner ink stream 32a than does a balanced air volume 42, the resulting reduction in air velocity near the end stream 32b arising from this greater separation causes the air pressure to be closer to its atmospheric value. The term xe2x80x9cinteractive forcexe2x80x9d is thus used to emphasize that forces F1, F2 interactively influence the ink steam and ink drop trajectories. These forces determine the shape of the air volumes between neighboring ink streams, which in turn influence the forces F1, F2 themselves.
Misalignment of ink drops in the slow scan direction can not be adequately corrected by compensating for aerodynamic drag using printing methods and printhead configurations that alter the ink drop velocity at end nozzles or provide for a later time of delivery for ink drops ejected from nozzles positioned proximate or at an end of the nozzle array. Additionally, adequate correction can not be obtained by other methods of compensating for aerodynamic drag, including displacement of end nozzles in the fast scan direction. This is especially evident in continuous ink jet systems having increased ink drop velocities and in inkjet systems having high density nozzle arrays.
Additionally, correcting misalignment of ink drops in the slow scan direction cannot be achieved by previous methods that compensate for ink drop misalignment caused by aerodynamic drag. For example, lower drop velocities are not sufficient to account for ink drop misalignment in the fast scan direction. It is however, important to correct for these problems, especially in high-density nozzle printing because, for example, in severe cases end drops may be so misaligned as to collide with drops ejected from neighboring nozzles before landing on the receiver. Accordingly, an apparatus and method of overcoming incongruent ink stream flow patterns at the end of the nozzle array in the fast scan and slow scan directions would be a welcomed advancement in the art.
An object of the present invention is to correct misdirection of ink streams and ink drops in a slow scan direction of an ink jet printhead.
Another object of the present invention to correct misdirection of ink streams and ink drops in a slow scan direction of an ink jet printhead having high nozzle densities.
Another object of the present invention is to provide a compensating or additional air sheath to correct misdirection of ink streams and ink drops.
Another object of the present invention is to prevent collisions between adjacent ink streams or ink drops prior to ink drops impinging on a recording medium.
Yet another object of the present invention to provide a high-density multiple nozzle array printhead having improved image resolution.
Yet another object of the present invention to provide a high-density multiple nozzle array printhead without the need for collinear air flow.
Yet another object of the present invention to provide a high-density multiple nozzle array with improved resolution without the need for permanently adjusting jet velocities of end nozzles.
Yet another object of the present invention to provide a means of high-density nozzle array design which simultaneously corrects misregistration in both the slow scan and fast scan directions providing improved resolution without need for permanently guttering the ink stream from the end nozzle.
According to an object of the present invention, an inkjet printing apparatus includes a source of ink and a printhead. The printhead has an end nozzle and a second nozzle adjacent to the end nozzle. A portion of the printhead is shaped to balance forces acting on the ink ejected from the end nozzle.
According to another object of the present invention, a printhead includes housing. Portions of the housing define a plurality of nozzle bores including an end nozzle bore and a second nozzle bore adjacent to the end nozzle bore. A portion of the housing is shaped to balance forces acting in a substantially s perpendicular direction relative to a path of ink ejected through the end nozzle bore and the adjacent nozzle bore as viewed from a plane substantially perpendicular to a plane defined by the ejected ink.
According to another object of the present invention, a method of balancing forces acting on ink ejected from an end nozzle includes providing a printhead having a plurality of nozzles including an end nozzle; and shaping a portion of the printhead such that forces acting on the ink ejected from the end nozzle are balanced, whereby ink drops formed from the ink ejected by the printhead are substantially equally spaced apart at a location removed from the printhead.